Supervolcanoes can erupt without an external trigger such as an earthquake, and may be more likely than previously believed, say scientists.
According to a study published in Nature Geoscience, the difference between supervolcanoes like Yellowstone and Lake Taupo and conventional volcanoes is how they erupt.
Normal volcanoes erupt when magma fills the underground chambers, and the pressure forces it out at the weakest point – the surface.
But supervolcanoes erupt when magma heats to a point where it is significantly less dense than the rock containing it. At this point it tries to escape, just like a balloon underwater. As supervolcano bases can cover hundreds of square kilometres, the resulting explosion isn't contained in a single exit point.
"Essentially we identify two different trigger mechanisms for eruptions - one for small ones up to about 500 cubic kilometres of magma, and one where we can generate super-eruptions," author Luca Caricchi, a volcanologist at the University of Geneva in Switzerland, told Nature.com.
They call it the 'bouyancy effect' – the magma literally lifts the rock above it until it cracks.
"The bigger a magma chamber gets, the more buoyancy will start to play in," says geoscientist Wim Malfait .
According to their calculations, the biggest a magma chamber could get would be about 35,000 cubic kilometres. This is about seven times larger than the explosion which created the La Garita Caldera in Colorado 26 million years ago, the largest eruption known to modern science.
That explosion was was 5000 times stronger than the most powerful hydrogen bomb ever detonated.
The pressure from buoyant magma could be enough to crack through more than 10km of the Earth's crust, and without needing the trigger of an earthquake, could happen with very little warning.
"The risk at any given time is small, but when it happens the consequences will be catastrophic," says Malfait.
Taupo's eruption around 1800 years ago remains the most violent the world has seen in the last few thousand years.
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